1. Field of the Invention
The present invention relates to a printing system, a printing method, an image processing system, and an image processing method for printing a patch that is measured by a sensor in order to obtain a measurement value.
2. Description of the Related Art
In recent years, many image output apparatuses, such as large ink-jet printers, equipped with a colorimetric device are in the marketplace. Such a colorimetric device is used for color calibration for correcting the print color density of a printer and stabilizing colors or to create an ICC profile. Colorimetric devices are mounted in various ways such as mounted on a carriage portion of the print head or provided in such a manner that a colorimetric device unit is attached and fixed to the paper ejection portion. Measurement methods of the colorimetric devices include contact and non-contact methods. In a contact method, measurement is performed while a colorimetric device is in contact with a paper sheet in order to eliminate the influence of outside light, and therefore the colorimetric device needs to stop on the paper sheet during the measurement. However, in a non-contact method, measurement can be performed by scanning a paper sheet without stopping. In Japanese Patent Laid-Open No. 2000-283852, a technique in which colors are measured while a non-contact sensor is scanning without having to stop the scanning head is proposed. The purpose of the invention described in Japanese Patent Laid-Open No. 2000-283852 is to perform color measurement of multiple color patches efficiently. A certain amount of time is required for the color measurement of a patch in the case where the patch is measured while the head is being moved in the width direction of a medium. Accordingly, color measurement can be performed without stopping the head by setting the long side of each patch to a length larger than the length obtained by multiplying the moving speed of the head by the time required for the color measurement by the head, and by shifting the color measurement start coordinates relative to the coordinates of the boundary of each patch.
Meanwhile, as a colorimetric device embedded in commercially available printers, a colorimetric device constituted by a sensor which is manufactured at relatively low cost compared to an ordinary expensive colorimetric device is often used. Although noise is always present in a signal received by a sensor, the noise does not pose a problem when the signal received by the sensor is sufficiently large with respect to the noise, that is, when the signal-to-noise (SN) ratio is large. However, for a high-optical-density patch, that is a dark patch, a signal to be received becomes small and the relative proportion of the noise components becomes large, and thus the SN ratio deteriorates. In particular, in an inexpensive sensor, since the SN ratio, which is the ratio of a signal value to noise, is small and thus the relative ratio of noise to a signal value becomes large, noise has a large impact on the signal value, thereby reducing the accuracy of measurement.
The countermeasures to this problem may include, for example, increasing the light quantity, and increasing the amplification factor. In the case of increasing the light quantity, problems such as a thermal problem and a change in the spectral characteristics of the light sources may occur and, in addition, a bright patch may cause signal saturation. Also in the case of increasing the amplification factor, a bright patch is likely to cause signal saturation and, in addition, the noise itself may be amplified. Further, a colorimetric sensor, in general, performs color measurement after calibrating white balance, however, since the color measurement needs to be performed under the same conditions as those used when the calibration is performed in order to prevent the above-mentioned error, changing the light quantity or the amplification factor for each patch during the color measurement may increase the colorimetric error.
Therefore, an averaging procedure is generally used as a typical technique. Random noise generated while the sensor performs measurement tends to converge to zero by averaging out in terms of time. That is, by increasing the measurement time and increasing the number of averaging operations in terms of time, the relative noise ratio can be lowered, whereby the accuracy can be improved.
FIGS. 1A and 1B explain the SN ratio in the case where patches are measured by a sensor. FIG. 1A illustrates signal values when a low-optical-density patch, that is a patch producing a high reflected signal value, is measured. Meanwhile, FIG. 1B illustrates signal values when a high-optical-density patch, that is a patch producing a low reflected signal value, is measured. Although the signal values contain random noise, the influence of the noise is small when the reflected signal values are large, as in FIG. 1A, and therefore the measurement can be performed within a relatively short measurement time t1. Multiple samplings are performed during the measurement time t1 and then the averaging procedure is performed. Meanwhile, since the influence of the noise is large when the reflected signal values are small, as in FIG. 1B, the measurement time t1 is not sufficient to ensure the accuracy. Therefore, a measurement time t2, which is longer than t1, is required. In this case, the averaging procedure is performed during the measurement time t2 by using more samplings than those in the case of FIG. 1A.
However, in order to converge the noise components to near zero in the averaging procedure, it is necessary to take a longer measurement time for darker patches, thereby taking a longer time to measure the whole patch chart. In the case where the sensor measures a patch chart while moving thereover, the measurement time for each patch needs to be adjusted to a period of time in which the darkest patch can be stably measured. However, if the length of each patch is adjusted to the length with which the darkest patch can be measured stably, there is a problem in that the total measurement time for measuring all the patches becomes longer.